In the course of a continuously increasing need for communication and mobility, transmission techniques using electromagnetic signals are of particular interest. The rapidly expanding field of mobile radio which operates, for example, using the Global System for Mobile Communications Standard (GSM), represents an important application of such transmission techniques. So-called base stations are used as connecting nodes in order to set up a connection between two mobile radio subscribers or one mobile radio subscriber and a communication subscriber in the fixed network. In order to ensure that simultaneous transmission and reception are guaranteed at all times on an existing connection the GSM Standard provides two separate frequency bands for transmitting and receiving the electromagnetic signals. For example, base stations using the GSM 900 Standard transmit in the 925 to 960 MHz frequency band, and receive in the 880 to 915 MHz frequency band. In order to transmit and receive the signals, a typical base station has a transmitting/receiving antenna in addition to transmitting and receiving amplifiers. This transmitting/receiving antenna in consequence has to cover the entire frequency range from 880 MHz to 960 MHz, that is to say has to have a minimum bandwidth of 80 MHz.
Dipole antennas or planar antenna structures, so-called patch antennas, are normally used for such applications. Both antenna types typically have one port for inputting the signals to be transmitted and for outputting the received signals. A so-called duplex filter or duplexer is used in order to use the antenna simultaneously for transmission and reception operation in adjacent transmitting and receiving frequency bands. The duplexer is essentially a frequency filter, in order to split the transmission and received signals between the transmission path and reception path on the basis of the frequency bands. For this purpose, the duplexer typically has three connections, one each for the antenna, the transmitter and the receiver. This results in separate ports at the duplexer for the connection of the transmitter and of the receiver (in this context, see also FIG. 8).
Since the signal levels of the transmitted and received signals are generally very different, the duplexer must have a very high attenuation for the transmitted frequencies at the receiver port, in order to avoid overdriving and blocking of the receiver, and thus a reduction in the receiver sensitivity. In addition, the received frequencies must also be heavily attenuated at the transmission port, since the so-called wideband noise from the transmitter may fall in the reception band. The losses in the duplexer resulting from the high level of mutual attenuation required in consequence lead in an extremely disadvantageous manner to a reduction in the effective transmission power, and to a reduction in the receiver sensitivity.
The joint use of a typical antenna structure with one port for transmission and reception operation also results in limitations regarding the line impedances used. The optimum impedances for coupling the antenna structure to the transmitter and to the receiver cannot be chosen independently of one another. Normally, the duplex filters do not carry out any impedance transformation, so that the antenna structure, transmitter and receiver have the same impedance. A real impedance of 50 ohms is frequently selected for all the ports, as a compromise.
A semiconductor is normally used as the power output stage in the transmission path, typically having a low output impedance, and the impedance is thus transformed up to 50 ohm by means of a matching network. Owing to the transmitter and receiver having the same impedance, direct noise matching between the antenna structure and the first amplifier stage is not possible in the reception path either. A matching network is typically likewise used in order to transform the impedance of the coupling to the antenna structure to the optimum source impedance to achieve a minimum noise factor. However, losses in the matching networks result in a further disadvantageous reduction in the reception sensitivity and the transmission power. In general, matching networks also have the disadvantage that, as additional components, they result in costs and a space requirement.
Further disadvantages result in particular for the use of planar patch antennas, since planar antenna structures have relatively narrow bandwidths, so that it is impossible to use them for applications with wide bandwidth requirements. In this case, it is disadvantageous that the bandwidth of a normal antenna structure with one port has to cover the entire frequency range from the lowest to the highest operating frequency, which entire range comprises at least the width of the sum of the bandwidths of the transmission and reception bands, that is typically even wider, however, since there is also a guard band between the transmission band and the reception band.
Although antennas having two ports are already known, these structures are dimensioned such that the same frequency range is output at both ports. In these antennas, two mutually orthogonal polarizations are output from the same resonator. When so-called "dual polarized" structures are used in transmitting/receiving systems, a duplex filter is then connected downstream of each of the two ports, which once again leads to the series of disadvantages described above.